1. Field of the Invention
The present invention relates to a process for manufacturing a sintered iron-copper base porous alloy which is used for production of an oil-impregnated sintered bearing applied for various motors, and a process for manufacturing an oil-impregnated sintered bearing.
2. Related Art
Many of oil-impregnated iron-base sintered bearings are made of iron-copper base alloy. The iron-copper base alloy includes iron-bronze alloy, iron-brass alloy and alloy further containing another component such as Ni, Co, P, Pb, solid lubricants such as graphite, molybdenum disulfide, and the like. Illustrating a manufacturing process of the iron-bronze base sintered alloy by way of example, a powder of reduced iron or an atomized iron powder is used as a raw material iron powder. And, to this raw material added are an electrolyzed copper powder and a tin powder, or a bronze alloy powder, as a raw material for the bronze alloy phase, and a very small amount of a molding lubricant as necessity arises, to prepare a mixed powder. The mixed powder is compression-molded to form a compact, and it is then sintered by heating it in a reductive gas atmosphere such as a mixed gas of nitrogen and hydrogen. The sintered body obtained is subjected to sizing and cutting processing, for adapting it to dimensional accuracy required for the bearing and adjusting the pore condition of its bearing surface, if necessary. It is further subjected to impregnating into the pores a lubricating oil having viscosity suitable for its use condition, obtaining an oil-impregnated sintered bearing.
The material constituting an oil-impregnated sintered bearing for motors is required to be an alloy which is excellent in sliding property and simultaneously has high oil impregnation capability, and the oil pressure of a lubricant film must be retained so that familiarity at the initial of operation is good and sliding friction is little. It is known that a bearing for a motor used in cold districts in particular generates squeaky noises at sliding when the permeability of the bearing is high.
In this connection, it is described in Japanese Patent Application Laid-Open (JP-A) No.S64-15522 that the permeability of a bearing is lowered by using a finer complex powder as a raw materials so that an oil film is easily formed on the sliding surface, and thereby, a lubricating oil is hardly escaped out from the sliding surface into the bearing. On the other hand, Japanese Patent Application Laid-Open (JP-A) No. H8-20836 discloses that an oil-impregnated sintered bearing, having an inner layer portion in which the porosity is 25% by volume or more and an outer layer portion in which the porosity is 16% by volume or less, is produced by forming an alloy layer having a different composition on the surface of a molded and sintered body, in order to provide an oil-impregnated bearing which has good wear resistance and familiarity. It is also described in the above document that the permeability is 30 Darcy or less. On the other hand, it is proposed in Japanese Patent Application Laid-Open (JP-A) No. 2003-120674 that, for an oil-impregnated sintered bearing for electric motors, a bearing being composed of a sintered alloy having an intercommunicating porosity of 20 to 30% and having a permeability of 6 to 50×10−11 cm2 is impregnated with synthetic oil having a kinematic viscosity of 61.2 to 74.8 mm2/s. It also discloses that squeaky noise is not generated even if it is operated in a cold district environment, and that a spongy-like reduced iron powder having a particle size of 145 meshes minus sieve is used as a raw material powder in manufacture of the oil-impregnated sintered bearing.
However, when a fine raw material powder is used in like manner as in the above-mentioned JP-A S64-15522, the intercommunicating porosity of a sintered bearing also decreases and oil storage capability is reduced. They are not advantageous from the viewpoint of durability. Moreover, when an outer layer portion having different materials is provided in like manner as in the above-mentioned JP-A H8-20836, probability of generating strains and the like between the layers is raised due to the difference of materials, and problems are caused for mechanical strength, durability and the like. It is not advantageous from the viewpoints of simplification of its manufacturing process and the like, either.
In general, there is a correlation between the intercommunicating porosity of a sintered porous alloy and its density. Specifically, when the density is lessened, the intercommunicating porosity is raised and oil storage capability also increases. However, the permeability is also increases. Therefore, discharge of the impregnated lubricating oil from the bearing to the sliding surface and escape from the sliding surface to its inside become large, and that makes difficult to maintain the oil pressure. For decreasing the permeability, it is important to reduce the opening of pores on the bearing surface. In order to do that, it is effective to control the degree of processing when the inner bore surface (namely, the bearing surface) of the sintered porous bearing is sized, in such a manner that the surface portion is appropriately densified. However, if the sintered porous alloy has rough pores, it is required to enlarge the processing degree of sizing, in order to make small the rough pores opened to the inner borer surface, and the sizing power easily affects into the depth of the bearing. This causes reduction of the intercommunicating porosity of the bearing. Therefore, reduction of the permeability by means of sizing should be limitedly utilized. On the other hand, if the compacting pressure is raised or if a finer powder is used as a raw material powder in order to make small the pores of the sintered porous alloy, the density of the sintered alloy increases and the intercommunicating porosity is reduced.
It is therefore necessary to take many factors into consideration for control of manufacture, in order to realize an oil-impregnated sintered bearing in which the permeability is reduced while requisite intercommunicating porosity is maintained. Moreover, without employing complicated steps, such a bearing cannot be easily manufactured while strength and durability are retained on the bearing as a whole.